 Solution

For collinear saturated layers, $M_{\mathrm{tot}}=2M{s}_p+M{s}_f$, where $s_p,s_f=\pm 1$. Thus parallel states have $M_{\mathrm{tot}}=\pm 3M$ and $R=R_P$, whereas antiparallel states have $M_{\mathrm{tot}}=\pm M$ and $R=R_{AP}>R_P$. The free layer switches in the smaller-field interval; the reference layer switches only at larger fields and its loop is shifted by exchange bias.

The plotted switching fields are illustrative rather than supplied measurements. In the normal read-head minor-loop range, the reference stays positive and only the $3M\leftrightarrow M$ pair is sampled. Rotational reversal and domain formation can round these ideal abrupt steps.